1. Field of the Invention
The present invention relates in general to a ferroelectric liquid crystal device. More particularly, it relates to such a ferroelectric liquid crystal device improved with respect to driving characteristics.
2. Description of the Prior Art
As conventional liquid crystal displays, displays utilizing twisted nematic liquid crystal materials are well known in the field. The displays of this type often exhibit cross-talk between adjacent pixels when driven in matrix configuration.
Displays of the active matrix type provided with thin film transistors for driving respective pixels are also known. Fabrication of the displays of this type, however, may present difficulties owing to low yield of transistors formed on substrate particularly when the size of the substrate is increased. Needs for relatively large equipment investment also make it difficult to proceed with this type.
A new type display device utilizing ferroelectric chiral, smectic C materials has been suggested by N. A. Clark et al, in U.S. Pat. No. 4,367,924. In this device, the smectic liquid crystal material is layered, and the layers 15 are aligned perpendicular to opposed surfaces of the cell as shown in FIG. 1. The liquid crystal molecules lie flat on the surfaces and are restricted at the surface to only two positions (i.e. first and second states) out of the cone of possible orientations that the chiral, smectic C state allows. The surface of the cell has to be closely spaced (about 1 to 3 .mu.m separation) so that the bulk of the sample follows the molecular orientation at the surface, thereby creating the two surface stabilized states. The influence of the surfaces also helps to suppress the helix of the chiral smectic C material so that the two states are not disrupted. The dipole moment of spontaneous polarization, namely c-director, points normal to the surfaces, up in the first state and down in the second state for example. Hence, the device can be switched between the two states with a pulsed electric field applied via electrode on the surface. The influence of the closely spaced surface causes the switched state to latch so that the state is maintained after the pulse is over, indicating memory characteristics.
The required close spacing between the surfaces presents difficulties in fabrication. This requirement is necessary in order not to form helical structure in the liquid crystal layers. The spacing has to be no wider than 5 times the helical pitch. Such close spacing makes it possible that the surfaces restrict the molecules of the liquid crystal material. This also means that the influence of the surface condition may be significant. For example, appropriate wave forms of driving signals applied to the liquid crystal layer are usually asymmetrical in order to conduct suitable switching between the first and second states. The suitable asymmetrical wave forms tend to substantially vary with age. Accordingly, the stability of switching of the new liquid crystal display is not so high.